1. Field of the Invention
The present invention relates to a semiconductor laser unit having a stripe structure.
2. Description of the Related Art
Conventionally, a wide-stripe structure is employed for realizing a high-power semiconductor laser device. In the wide-stripe structure, the active layer is formed to have a width of more than 10 micrometers to increase the output power, while the width of the active layer in a usual single-mode laser device is about 3 micrometers. Therefore, a number of high-order transverse modes are mixed in oscillated light, and when the oscillation power is increased, the mode of oscillated light is liable to change to a different mode due to spatial hole burning of carriers, which is caused by high density distribution of photons in the resonant cavity. At the same time, near-field pattern, far-field pattern, and oscillation spectrum vary. In addition, the optical output power also varies due to the difference in efficiency of current-to-light conversion. This phenomenon is called a kink in the current/optical output power characteristic of a semiconductor laser device.
Therefore, when the above high-power semiconductor laser device is used as an excitation light source in a solid-state light emitting apparatus, a laser-diode-excited SHG solid-state laser, or a light emitting apparatus to which an optical fiber is connected, the following problems arise.
When the above high-power semiconductor laser device is used as an excitation light source in a solid-state laser apparatus, only a component coupled with an oscillation mode of the solid-state laser resonator is utilized as an excitation light from among oscillated light generated by the semiconductor laser device and condensed by a lens system. Therefore, the output intensity greatly varies with change of the transverse mode. In addition, since the absorption spectrum of the solid-state laser has a fine absorption spectrum structure in a narrow wavelength band, an amount of absorbed light varies in response to the variation of an oscillation spectrum. Thus, the output intensity of the solid-state laser apparatus is further affected by the variation of the oscillation spectrum, in addition to the change of the transverse mode. Further, use of a spatial or spectral portion of the light generated by the solid-state laser device increases high-frequency noise accompanied by switching between transverse modes.
The laser-diode-excited SHG solid-state laser is a visible-range light emitting apparatus in which a wavelength of a fundamental wave generated by a solid-state laser apparatus is converted to a half of the wavelength by using a nonlinear crystal to generate a second harmonic wave. When the high-power semiconductor laser device is coupled to a solid-state laser crystal or a nonlinear crystal for generating a second harmonic wave, the above noise is further increased due to the nonlinear effect.
When the high-power semiconductor laser device is coupled to an optical fiber, the optical fiber is used as an output end. Therefore, it is possible to separate the semiconductor laser device which needs heat dissipation, and miniaturize the light source portion. In addition, since an optical fiber cuts off components of light other than a component in the propagation mode, the optical fiber functions as a mode filter to improve optical quality.
Nevertheless, when the semiconductor laser device and the optical fiber are coupled, the above-mentioned problems of the variation of the output intensity due to the change of the transverse mode in the semiconductor laser device and the noise due to the switching between transverse modes also arise. In particular, in applications for generating high-quality images, the image generating operation is impeded by noise even when the noise is at a level of about 1 percent. However, conventional high-power semiconductor laser devices cannot achieve such a severe noise requirement.
In the high-power semiconductor laser devices, the above variation of the output intensity and noise caused by the transverse-mode oscillation can be reduced by disposing materials having refractive indexes different from that of an active layer on both sides of the active layer in the direction of the width of the active layer, i.e., in the lateral (transverse) direction, and arranging an index-guided waveguide structure for achieving confinement of optical waves in the transverse modes. However, if the effect of the confinement is too great, the increase in the photon density in the active layer causes catastrophic optical damage (COD) to decrease the output intensity. Otherwise, if the effect of the confinement is too small, the efficiency of the current-to-light conversion decreases, and therefore the output intensity decreases.
In addition, conventionally, it is difficult to suppress the noise to several percent even when the above index-guided waveguide structure is used.
The object of the present invention is to provide a short-wavelength semiconductor laser unit having a stripe structure, achieving low-noise oscillation and providing a stable optical output.
The object of the present invention is accomplished by the present invention, which provides a semiconductor laser unit containing a semiconductor laser device and a heat sink to which the semiconductor laser device is bonded. The semiconductor laser device contains a stripe structure having a width equal to or greater than 10 micrometers, and including a first optical guide layer of a first conductivity type, an active layer, and a second optical guide layer of a second conductivity type. A total thickness of the first and second optical guide layers is equal to or more than 0.5 micrometers. The semiconductor laser device is soldered onto the heat sink at a surface of the semiconductor laser device, where the surface is located farther from the active layer than other surfaces of the semiconductor laser device. That is, the semiconductor laser device is bonded to the heat sink so as to form a so-called junction-up configuration.
Since, according to the present invention, the optical guide layers, which guide light waves, are formed to have a total thickness not less than 0.5 micrometers in the stripe structure having a thickness not less than 10 micrometers, and photon density in the light-emitting area is made small, it is possible to prevent spatial hole burning of carriers due to the high photon density distribution, and reduce the variation of the optical output due to the change of the transverse mode. Thus, a stable optical output can be obtained.
In addition, since the photon density is made small, it is also possible to prevent the occurrence of catastrophic optical damage (COD). This also makes the optical output stable.
Further, conventionally, a semiconductor laser device or the like is soldered onto a heat sink to form the so-called junction-down configuration. That is, conventionally, the semiconductor laser device is bonded to the heat sink at a surface, which is located near the active layer, of the semiconductor laser device. On the other hand, according to the present invention, the semiconductor laser device is soldered onto the heat sink to realize the so-called junction-up configuration. That is, the semiconductor laser device is bonded to the heat sink at a surface, which is located far from the active layer, of the semiconductor laser device. Therefore, it becomes possible to reduce an influence, on the active layer, of the strain generated at the bonded surfaces due to the difference in thermal expansion coefficients between the semiconductor laser device and the heat sink. Thus, noise can be reduced.